This invention relates to a process for the preparation of pyrimidinone compounds of the formula ##STR1## wherein Ar is an optionally substituted aryl or heteroaromatic moiety,R.sup.3 is an optionally substituted alkyl, alkenyl or alkynyl,R.sup.5 is a hydrogen atom, halo, cyano or an optionally substituted alkyl, alkenyl, alkynyl, alkoxy or alkylthio, andR.sup.6 is a hydrogen atom, cyano, or an optionally substituted alkyl, cycloalkyl, alkenyl, alkynyl, aryl, aralkyl or heteroaryl.The process comprises reaction of an amidine and a malonic acid derivative to give a hydroxypyrimidinone followed by conversion of the resulting hydroxypyrimidinone to a sulfonyloxypyrimidinone followed by reaction of the resulting sulfonyloxypyrimidinone with an organometallic reagent to give the desired pyrimidinone. The hydroxypyrimidinones, wherein R.sup.6 is hydroxy and sulfonyloxypyrimidinones wherein R.sup.6 is a substituted sulfonyloxy moiety, are themselves new and useful intermediates in the preparation of the desired pyrimidinones.